Increasing levels of integration of integrated circuit chips reduces the chip count of a functional circuit, while significantly increasing the I/O count of the individual integrated circuits making up the functional circuit. This drive for increased circuit and component density in the individual integrated circuit chips leads to a parallel drive for increased circuit and component density in the printed circuit boards carrying the chips and in the assemblies using them.
The increased circuit and component density in the printed circuit boards makes the ability to locate either solder surface mount components or place additional circuitry layers directly above plated through holes highly desirable. This is especially the case when the density of the plated through holes required to service the I/O's of the surface mount components is such that there is no surface area available for attachment pads interstitial to the plated through hole grid.
The problem is especially severe with fine pitch ball grid array components (BGA) and flip chip attach integrated circuits. Soldering of these surface mount components to the surface pads, i.e., lands, of conventional plated through holes is highly undesirable. This is because the solder used for assembly tends to wick down into the plated through holes. The result is low volume, unreliable solder joints.
One solution that has been proposed is filling the plated through holes. However, known methods of filling plated through holes of printed circuit boards have deficiencies. For example, they suffer from bleed of the resin component of the PTH fill material along the surface of the boards. This resin also bleeds into holes which are not to be filled. This leads to short circuits and to soldering defects during assembly.